Field
The present systems and methods generally relate to cryogenic refrigeration technology.
Refrigeration
Temperature is a property that can have a great impact on the state and evolution of a physical system. For instance, environments of extreme heat can cause even the strongest and most solid materials to melt away or disperse as gas. Likewise, a system that is cooled to cryogenic temperatures may enter into a regime where physical properties and behavior differ substantially from what is observed at room temperature. In many technologies, it can be advantageous to operate in this cryogenic regime and harness the physical behaviors that are realized at low temperatures. The various embodiments of the systems, methods and apparatus described herein may be used to provide and maintain the cryogenic environments necessary to take advantage of the physics at cold temperatures.
Throughout this specification and the appended claims, the term “cryogenic” is used to refer to the temperature range of 0 Kelvin (K) to about 93K. A variety of technologies may be implemented to produce an environment with cryogenic temperature, though a commonly used device that is known in the art is the helium-3-helium-4 dilution refrigerator, known as, a dilution refrigerator. Dilution refrigerators achieve extreme cryogenic temperatures below 50 mK. In the operation of a typical dilution refrigerator, the apparatus itself requires a background temperature of about 4K. In order to provide this background cooling, the apparatus may be, e.g., immersed in an evaporating bath of liquid helium-4 (4He) or, e.g., coupled to another type of refrigeration device, such as a pulse-tube cryocooler. The dilution refrigerator apparatus may comprise a series of heat exchangers and chambers that allow the temperature to be lowered further to a point where a mixture of helium-3 (3He) and 4He separates into two distinct phases. In the first phase is mostly 3He, known as the concentrated phase, and in the second phase is mostly 4He with some 3He, known as the dilute phase. The dilution refrigerator apparatus is configured to allow some of the 3He to move from the concentrated phase into the dilute phase in an endothermic process analogous to evaporation, providing cooling and allowing a temperature of around 10 mK to be achieved. The 3He is drawn out of the dilute phase mixture to through a counter-flow heat exchanger, condensed, cooled, returned to the concentrated phase portion of the mixture via the counter-flow heat exchanger to define a helium circuit. Even though the dilute phase is 4He rich the 3He is preferentially drawn from the dilute phase because 3He has a higher partial pressure than 4He. Further details on this dilution effect and the operation of typical dilution refrigerators may be found in F. Pobell, Matter and Methods at Low Temperatures, Springer-Verlag, Second Edition, 1996, pp. 120-156.
In most dilution refrigerator designs, mechanical pumps and compressors, and an external gas-handling system, are used to circulate 3He such that it is warmed from the lowest temperature in the fridge up above cryogenic temperatures and towards room temperature before it is returned to the low temperature. The pumps and compressors used are large, expensive, noisy, in need of periodic maintenance, and they inevitably add contaminants, such as air (i.e., nitrogen, oxygen, carbon dioxide, argon, etc.) to the helium. These contaminants typically have higher freezing points than the helium and so may solidify in the helium fluid channels, creating blockages. Such blockages may plug fine capillaries in the dilution refrigerator, causing problems with reliability. Plugging often requires a complete warm-up of a dilution refrigerator in order to remove the contaminants and restore the fridge to normal operations. The procedure of warming and subsequently cooling back down to operating temperatures can take several days. Filters and cold traps can be used to reduce the frequency of plugging by removing contaminants from the helium, but current filters and traps are of limited effectiveness. Thus, plugging due to contaminants, such as, nitrogen, oxygen, carbon dioxide, and argon remains a serious technical challenge in cryogenic refrigeration technology affecting refrigeration system performance and availability.